Transistors can be employed as switches in electrical circuits. In particular, power MOSFETs are employed as switches in a load circuit for coupling a load to a source. The transistor may be either switched on, i.e., to its conducting state, thus allowing a current flow through the load circuit, or the transistor may be switched off, i.e., switched to non-conducting.
When switching the transistor on, i.e., switching the transistor to conducting, the electrical load is coupled to the voltage or current source allowing a current to flow through the circuit, i.e., through the transistor and the load. When switching a transistor off, i.e., switching the transistor from conducting to non-conducting, the transistor will disconnect the load from the source, such that the transistor will stop a current flow in the load circuit. However, if the load circuit comprises an inductor, the current flow in the load circuit will not stop immediately when switching the transistor off. An inductor included in the load circuit will discharge its stored energy, such that the current flowing in the load circuit and through the transistor decreases with time until the inductor is fully discharged.
The transistor may be designed to stand the current flow, i.e., the dimensions of the transistor are chosen such that under normal operating conditions the current will not cause any damage to the transistor.
For higher currents, a plurality of transistors can be switched in parallel, such that each of the plurality of transistors carries only a part of the total current. Although the current in this case is distributed across the plurality of parallel transistors, each of the plurality of transistors is designed for carrying only a portion of the total current. The problem of overload thus remains for each transistor.
However operating conditions may deviate from normal operations, for which the transistor has been designed, and a higher current may flow through a transistor. For example, in case of a short on the load side of the transistor, an unusually high current may flow, causing a current density in the transistor exceeding the allowed range.
A current causing a current density exceeding the allowed range may destroy the transistor by heating the semiconductor structure, such that the transistor is locked in its conducting state and cannot be switched off by applying an appropriate gate voltage. In this case the transistor is destroyed.
Hence a transistor operated as a power switch should be protected from situations heating the transistor above an allowed temperature.
In most cases where the transistor is switched off, it will be later switched to conducting in order to couple the load to the power supply again. In case the overload situation will occur again the transistor will be repeatedly switched on and off. Hence there is a need for an improved method and circuit for minimizing the duration when the transistor is switched off while at the same time preventing the transistor from being heated beyond its limit.